1. Field of the Invention
This invention relates to an apparatus used for metal organic chemical vapor deposition (hereinafter, abbreviated as MOCVD) and a method using the same. Particularly, this invention concerns the apparatus and the method for growing a compound semiconductor layer on a substrate. This invention has a particular applicability in connection with an apparatus, in which a metal organic gas and a reactive gas are source material gases and a hydrogen gas is used as a dilution gas.
2. Description of the Related Art
An MOCVD method is widely used when a compound semiconductor such as a group III-V or II-VI binary, ternary, or quaternary semiconductor is grown on a semiconductor substrate. In the MOCVD method, a compound gas composed of metal and methyl (or ethyl) group is used as one of source material gases, and is fed to a reactor chamber bubbled with a hydrogen gas. MOCVD is sometimes called MOVPE (metal organic vapor phase epitaxy), however, a word "MOCVD" is used hereinafter in the present invention.
A horizontal or vertical type is used for a reactor chamber in the MOCVD method. In FIG. 1, an MOCVD apparatus is schematically shown when a vertical type reactor chamber is used. A horizontal type reactor chamber is more suitable than the vertical type when plural wafers are processed at a time. However, when uniformity in thickness or composition of a grown compound semiconductor layer is important, the vertical type MOCVD apparatus has many advantages for these purposes
In FIG. 1, a plurality of source material gases and a hydrogen dilution gas are mixed and fed into a reactor chamber 5 through a main valve V.sub.m and a vent pipe 1. The vent pipe 1 is sealed to an upper wall portion of the reactor chamber 5 and broadens toward its opening 101. A wafer or a substrate 2 is loaded on a susceptor 8, which is rotated around a support axis 7 during operation, and the substrate is located at position opposed to the opening 101. The vent pipe 1 has a reversed funnel-like shape and functions to make gas flow more uniform onto the surface of the substrate 2. The susceptor 8 is made of carbon material and is heated by a high frequency induction coil 17 in order to raise a substrate temperature up to a specified value. The used gas is finally discharged through an opening 6.
A plurality of material gas sources 41 to 43 is provided and mixed through valves V.sub.1, V.sub.2 and V.sub.3. In FIG. 1, three source material gas sources are representatively shown. Generally, a hydrogen dilution gas source 40 is provided and the hydrogen (H.sub.2) gas is supplied through a valve V.sub.0 and mixed with source material gases in order to control gas flow rate and obtain uniformly mixed gas. Each source material gas is provided in either a bombe containing a source material gas or in a bubbler containing a metal organic compound source in a liquid or solid phase which is bubbled by a hydrogen gas. For example, when a ternary semiconductor GaAsP or InGaAs is grown on a binary semiconductor substrate of GaAs or InP respectively, three material gas sources are provided. As an indium gas source, trimethylindium (In(CH.sub.3).sub.3) is provided in a bubbler, and as a gallium gas source, trimethylgallium (Ga(CH.sub.3).sub.3) is provided in a bubbler. An arsenic or phosphorus gas source, such as arsine [AsH.sub.3 ] or phosphine [PH.sub.3 ] gas, is provided in a bombe.
In order to use these metal organic gas sources, a bubbler containing the metal organic compound is bubbled with a hydrogen gas. The quantity of the metal organic compound output from the bubbler is controlled by an inside temperature of the bubbler and the flow rate of the bubbling hydrogen gas. On the other hand, the arsine or phosphine gas from the bombe is also mixed with a hydrogen gas in order to increase a flow velocity in pipe lines. In growing a III-V group semiconductor layer, a ratio of the V group source material to the III group source material in the mixed gas is maintained at a suitable value in order to obtain a proper growth rate and a proper mixing ratio of the grown mixed semiconductor, and these factors change depending on the kinds of the semiconductor layer to be grown. In order to control the growth process easily and obtain a uniform mixture of source material gases, the mixed gas of these source material gases is further diluted by a hydrogen gas from the dilution gas source 40 and thereafter fed to the reactor chamber 5.
The most important factor in growing a compound semiconductor layer on a substrate is that a uniform thickness of the grown layer is obtained over the entire surface of the substrate and further, when the grown semiconductor is a mixed compound semiconductor, the mixing ratio thereof is also uniform on the substrate surface as much as possible. The latter condition can be expressed in other words that the grown semiconductor, for example, In.sub.1-x Ga.sub.x As or In.sub.1-x Ga.sub.x As.sub.y P.sub.1-y has an almost constant x-value or (x, y)-value over the entire substrate surface.
However, in an application of the MOCVD apparatus shown in FIG. 1, though it has the vent pipe 1 which has the enlarged opening 101 at a spouting side of gas into the reactor chamber in order to get more uniform gas flow onto the substrate surface, it is a difficult matter to get a uniform thickness of the grown layer over the entire substrate surface and also a uniform composition thereof. To improve the uniformity of gas flow, a vent pipe 1 shown in FIG. 2 is proposed. The vent pipe 1 has plural branched pipes 3 at the spouting side thereof. The MOCVD apparatus utilizing the vent pipe such as shown in FIG. 2 is effective to some extent, however, it needs a vent pipe 1 with branched pipes 3 having a different diameter and a different shape. And moreover, when the gas flow rate, the growth temperature, etc. or the kinds of gases are changed, another type of the vent pipe must be provided, which has branched pipes with a different diameter in order to get uniform growth on the substrate. It is a difficult and practically impossible matter to change the vent pipe during the growing process in order to obtain a different semiconductor layer.
Other improvements for obtaining uniformity of the grown semiconductor layer, especially, with regard to the vertical type MOCVD apparatus are disclosed in the following Japanese Unexamined Patent Publication: Nos. SHO-58-176196 dated Oct. 15, 1983, by A. Takamori et al. SHO-60-189928 dated Sept. 27, 1985, by Y. Shiotani
The above two disclosures are related to an MOCVD apparatus in which two gas sources are separately introduced into a reactor chamber and spouted from plural distributed nozzles or branched vent pipes and mixed in the neighborhood of the substrate. These disclosures have a main object of avoiding a mutual reaction between two source material gases before they are spouted into the reactor chamber. However, there still remain problems of making the gas flow rate, the mixing ratio of gases, the density of the mixed gas, etc. more uniform.